Answer: near the center of Earth
Explanation:
Earth is the only planet in the solar system with an atmosphere that can sustain virtually all life, this is due to it diverse regional climates.
Assuming there is symmetrical or proportionate distribution of the Earth atmosphere and it is a perfect sphere, one can say that the center of mass of the Earth atmosphere will be at the center of spherical Earth, But since it's not a perfect sphere, the center mass of the earth atmosphere is near the center of the earth.